Well some good subs are the MTX 8000 series, Alpine Type R's, JL Audio W3's, and Infinity Perfects...some good amps to power the subs(2) would be JBL 1200.1 and the MTX 81000D....can't help you with speakers because you havent given sizes but I reccomend the Infinity Kappa series, MB Quarts, and Boston Acoustics....Phoenix Gold and MTX make good 4 channel amps.

Boston Rally speakers should (not all, most) fit in your front door cabinets, if i'm right the WRX also has "rear" speakers that are in cabinets fairly high in the backside of the car.. Boston also has fitting speakers for that, also pioneer has fittable speakers (not back, front yes)

Altho you should know that its mad to install it yourself in a new subaru.. .i strongly advice you to let it be done by a certified and trusted dealer

you have 6.5"'s in the front doors and 4"'s in teh rear doors...
first, lets deal with the subs...
are you going for pure earth shattering volume, or crisp clear bass? or a mix of teh two?
for the speakers inside, all those mentioned are excellent choices, but i would go with a 6.5" components system, so basically a wofoer int eh door, and tweeters mounted someone else...
are you willing to put in kick panels? if so, best choice.......